Closet Veggie-Lovers?: 'Predatory' Dinos Ate Plants

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Dinosaurs long thought to be predatory have been outed as
plant-eaters by their anatomical features — including birdlike
beaks.

A new analysis of Coelurosauria, the group of theropod dinosaurs
that includes
bruisers such as T. rex, finds that dedicated
hunters were likely the exception, rather than the rule. The
findings, published the week of Dec. 20 in the journal
Proceedings of the National Academy of Sciences, suggest that
famous Coelurosaur predators likely had some veggie-lovers in the
family tree.

"Anything that belongs in that group that seems to be an
exclusive meat-eater — things like
Velociraptor of 'Jurassic Park' fame — it means it
probably evolved from an ancestor that had already begun eating
some plants," study author Lindsay Zanno of the Field Museum in
Chicago told LiveScience.

The anatomy of a plant-eater

Coelurosaurs are the subset of dinosaurs most closely related to
birds. They've long been considered hunters, Zanno said, but
recent fossil discoveries revealed Coelurosaurs with peg-like
teeth or beak-like mouths. While the knife-like
chompers of a T. rex or Velociraptor were obviously
for tearing meat, these new finds are ambiguous. After all, birds
today eat almost everything with their beaks, from berries to
carrion.

Zanno and her Field Museum colleague Peter Makovicky started by
collecting what little firm dino-diet data they could find. They
identified 72 Coelurosaur species that had been found with direct
fossilized evidence of what they ate, including fossilized
stomach contents and feces. Then the researchers statistically
analyzed those dinosaurs' anatomy, looking for physical traits
that correlated with plant-eating.

They found 21 markers of a plant-eater, including a longer neck,
leaf-shaped or peg-like teeth, and the evolution of a beak. When
the researchers applied the findings to the rest of the
Coelurosaur family, they found 44 species across six subgroups
that likely ate at least some plant matter, judging by their
anatomy.

"It appears as if the transition from carnivory, or meat-eating,
to plant-eating occurred very early in the evolutionary history
of the group and probably occurred once," Zanno said. That means
that dinosaurs like T. rex and Velociraptor likely
evolved back to meat-only diets after their ancestors spent at
least some time chowing on plants.

Herbivore origins

"We're not seeing special cases of herbivory evolving from
carnivorous ancestors," said Thomas Holtz, a vertebrate
paleontologist at the University of Maryland who was not involved
in the study. "We're seeing one origin of herbivory here."

The find raises new questions about the ecological niches
Coelosaurs inhabited, Holtz said, such as whether switching to
plant-based diets restricted their range.

The study is also important for understanding the evolutionary
history of birds, Zanno said. Although beaks evolved
independently many times throughout history, the new study
suggests that a plant-based diet was the push Coelosaurs needed
to turn toothy mouths into beaks.

"Once you have a beak, you can adapt it into all different shapes
and sizes, and you can adapt it to do all different kinds of
things," Zanno said.